Biologically inspired surfaces are typically modeled after the surface properties of one particular species or combine those of multiple biological species to create a cross-species surface. Among the latter types of bioinspired surfaces, those that selectively switch between different functional states are rare.
From the exceptional camouflage capabilities of the cuttlefish and chameleon to the changeable wettability of the honeybee tongue, physical materials that can change their interfacial properties on-demand allow them to interact with their dynamic environments effectively. While materials with dynamic optical properties can be readily found in nature, natural surfaces that can dynamically alter their liquid-repellent functions are rare. Rather, while there are various liquid-repellency strategies among different natural species, all of them rely on static surface textures.
Synthetic liquid-repellent surfaces are primarily modeled after two classes of biological surfaces. The first class of surfaces, known as superhydrophobic surfaces, relies on air-infused solid textures to repel impinging liquid droplets. These surfaces are modeled after natural surfaces such as lotus leaves and springtails. The second class of surfaces, called slippery liquid-infused porous surfaces (SLIPS), utilize liquid-infused solid textures to repel immiscible fluids in a manner similar to that of the pitcher plant peristome. See T. S. Wong, et al. Nature 2011, 477, 443.
Owing to the presence of an air-layer, superhydrophobic surfaces are known for their excellent self-cleaning properties and high droplet mobility. However these surfaces often fail to maintain these properties when under significant pressure, or in high humidity environments, or at elevated temperatures. On the other hand, SLIPS are known for their exceptional liquid repellency in extreme pressure, temperature, or humidity conditions. However, these surfaces typically display lower droplet shedding speeds compared to their superhydrophobic counterparts. See D. Daniel, et al., Appl. Phys, Lett 2013, 102, 231603.
While synthetic surfaces with switchable wettability have been proposed (see X. Yao, et al., Nat. Mater 2013, 12, 529, none of these surfaces are capable of dynamically switching between superhydrophobic and SLIPS (hereafter “slippery”) modes. A surface that can dynamically switch between such modes could maintain liquid-repellency over a broad range of environmental conditions, and could display high droplet mobility when environmental conditions allow.